Isolated switched-mode direct-current (DC) to DC power converters use a transformer to convert power from an input source into power for an output load. Such power converters include power switches that convert DC input power into alternating current (AC) power that is fed to the primary side of the transformer. AC power supplied on the secondary side of the transformer is rectified to convert it back into DC power which is filtered and provided to the output load. The primary-side power switches are typically controlled by pulse-width-modulated (PWM) control signals. A controller generates the PWM control signals with a frequency and duty cycle that are appropriate to meet the power needs of the output load.
The controller typically uses a linear closed-loop feedback technique to maintain the output voltage near a desired target. The controller may be implemented using analog or digital circuitry, and may be located on the primary or secondary side of the power converter. So as to maintain the integrity of the isolation barrier of the power converter, any signals crossing between the primary and secondary sides must pass through isolators, e.g., transformers, opto-couplers. Analog controllers are often preferably located on the primary side of the power converter, where the input voltage and/or current may be readily sensed and used to augment the closed-loop control. However, such primary-side controllers have the disadvantage that one or more analog isolators must be used to transfer the output voltage and/or current information from the secondary to the primary side for purposes of implementing the closed-loop control. Furthermore, system managers are typically located on the secondary side, meaning that communication signals from a system manager must also pass through isolators. For these reasons, as well as cost, size, and flexibility advantages, isolated switched-mode power converters increasingly use digital controllers that are located on the secondary side.
Closed-loop control provides good regulation of the output voltage for a power converter when the input provides stable power and the load current is fairly constant, or if a large output capacitor is used to maintain good transient response. As switched-mode power converters decrease in size, e.g., due to increasing switching frequencies and smaller inductors, the output capacitor is consuming a larger relative portion of the power converter size, so there is a significant trend towards reducing the output capacitor size. For a given output capacitance, the linear closed-loop control design for a power converter represents a trade-off between fast transient response and loop stability. So as to provide good stability and fast transient response while using smaller output capacitors, enhanced control techniques may be used to supplement conventional linear feedback control. Such enhanced control techniques may be used, e.g., to more quickly adjust to load transients or input voltage transients.
Feedforward control techniques may monitor the input voltage of a power converter so as to detect input voltage transients (changes) that a linear closed-loop feedback control technique might not be able to adequately track. Such detection may then be used to augment the closed-loop control so as to quickly respond to the input transient. Because such feedforward compensation typically requires tracking (measuring) the input voltage, feedforward compensation is often difficult or unfeasible to implement within a digital controller located on the secondary side of a power converter, as the input voltage is not readily available to such a controller.
Power converter control techniques that quickly compensate for input transients and that are readily implemented within a secondary-side controller are desired. These techniques should require little or no additional circuitry beyond that required for closed-loop feedback control.